Duenna

[Lady ]. The female of don. The Spanish don is
derived from the Latin dominus =a lord, a master. A duenna is
the chief lady-in-waiting on the Queen of Spain; but in common
parlance it means a lady who is half companion and half governess, in
charge of the younger female members of a nobleman's or gentleman's
family in Portugal or Spain.

“There is no duenna so rigidly prudent and inexorably decorous as a
superannuated coquette.” —W. Irving: Sketch-Book (Spectre
Bridegroom).